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<a name="Register-Basics"></a>
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Next: <a href="Allocation-Order.html#Allocation-Order" accesskey="n" rel="next">Allocation Order</a>, Up: <a href="Registers.html#Registers" accesskey="u" rel="up">Registers</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Basic-Characteristics-of-Registers"></a>
<h4 class="subsection">18.7.1 Basic Characteristics of Registers</h4>

<p>Registers have various characteristics.
</p>
<dl>
<dt><a name="index-FIRST_005fPSEUDO_005fREGISTER"></a>Macro: <strong>FIRST_PSEUDO_REGISTER</strong></dt>
<dd><p>Number of hardware registers known to the compiler.  They receive
numbers 0 through <code>FIRST_PSEUDO_REGISTER-1</code>; thus, the first
pseudo register&rsquo;s number really is assigned the number
<code>FIRST_PSEUDO_REGISTER</code>.
</p></dd></dl>

<dl>
<dt><a name="index-FIXED_005fREGISTERS"></a>Macro: <strong>FIXED_REGISTERS</strong></dt>
<dd><a name="index-fixed-register"></a>
<p>An initializer that says which registers are used for fixed purposes
all throughout the compiled code and are therefore not available for
general allocation.  These would include the stack pointer, the frame
pointer (except on machines where that can be used as a general
register when no frame pointer is needed), the program counter on
machines where that is considered one of the addressable registers,
and any other numbered register with a standard use.
</p>
<p>This information is expressed as a sequence of numbers, separated by
commas and surrounded by braces.  The <var>n</var>th number is 1 if
register <var>n</var> is fixed, 0 otherwise.
</p>
<p>The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either automatically,
by the actions of the macro <code>CONDITIONAL_REGISTER_USAGE</code>, or by
the user with the command options <samp>-ffixed-<var>reg</var></samp>,
<samp>-fcall-used-<var>reg</var></samp> and <samp>-fcall-saved-<var>reg</var></samp>.
</p></dd></dl>

<dl>
<dt><a name="index-CALL_005fUSED_005fREGISTERS"></a>Macro: <strong>CALL_USED_REGISTERS</strong></dt>
<dd><a name="index-call_002dused-register"></a>
<a name="index-call_002dclobbered-register"></a>
<a name="index-call_002dsaved-register"></a>
<p>Like <code>FIXED_REGISTERS</code> but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers.  This macro therefore identifies the registers that are not
available for general allocation of values that must live across
function calls.
</p>
<p>If a register has 0 in <code>CALL_USED_REGISTERS</code>, the compiler
automatically saves it on function entry and restores it on function
exit, if the register is used within the function.
</p>
<p>Exactly one of <code>CALL_USED_REGISTERS</code> and <code>CALL_REALLY_USED_REGISTERS</code>
must be defined.  Modern ports should define <code>CALL_REALLY_USED_REGISTERS</code>.
</p></dd></dl>

<dl>
<dt><a name="index-CALL_005fREALLY_005fUSED_005fREGISTERS"></a>Macro: <strong>CALL_REALLY_USED_REGISTERS</strong></dt>
<dd><a name="index-call_002dused-register-1"></a>
<a name="index-call_002dclobbered-register-1"></a>
<a name="index-call_002dsaved-register-1"></a>
<p>Like <code>CALL_USED_REGISTERS</code> except this macro doesn&rsquo;t require
that the entire set of <code>FIXED_REGISTERS</code> be included.
(<code>CALL_USED_REGISTERS</code> must be a superset of <code>FIXED_REGISTERS</code>).
</p>
<p>Exactly one of <code>CALL_USED_REGISTERS</code> and <code>CALL_REALLY_USED_REGISTERS</code>
must be defined.  Modern ports should define <code>CALL_REALLY_USED_REGISTERS</code>.
</p></dd></dl>

<a name="index-call_002dused-register-2"></a>
<a name="index-call_002dclobbered-register-2"></a>
<a name="index-call_002dsaved-register-2"></a>
<dl>
<dt><a name="index-TARGET_005fFNTYPE_005fABI"></a>Target Hook: <em>const predefined_function_abi &amp;</em> <strong>TARGET_FNTYPE_ABI</strong> <em>(const_tree <var>type</var>)</em></dt>
<dd><p>Return the ABI used by a function with type <var>type</var>; see the
definition of <code>predefined_function_abi</code> for details of the ABI
descriptor.  Targets only need to define this hook if they support
interoperability between several ABIs in the same translation unit.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fINSN_005fCALLEE_005fABI"></a>Target Hook: <em>const predefined_function_abi &amp;</em> <strong>TARGET_INSN_CALLEE_ABI</strong> <em>(const rtx_insn *<var>insn</var>)</em></dt>
<dd><p>This hook returns a description of the ABI used by the target of
call instruction <var>insn</var>; see the definition of
<code>predefined_function_abi</code> for details of the ABI descriptor.
Only the global function <code>insn_callee_abi</code> should call this hook
directly.
</p>
<p>Targets only need to define this hook if they support
interoperability between several ABIs in the same translation unit.
</p></dd></dl>

<a name="index-call_002dused-register-3"></a>
<a name="index-call_002dclobbered-register-3"></a>
<a name="index-call_002dsaved-register-3"></a>
<dl>
<dt><a name="index-TARGET_005fHARD_005fREGNO_005fCALL_005fPART_005fCLOBBERED"></a>Target Hook: <em>bool</em> <strong>TARGET_HARD_REGNO_CALL_PART_CLOBBERED</strong> <em>(unsigned int <var>abi_id</var>, unsigned int <var>regno</var>, machine_mode <var>mode</var>)</em></dt>
<dd><p>ABIs usually specify that calls must preserve the full contents
of a particular register, or that calls can alter any part of a
particular register.  This information is captured by the target macro
<code>CALL_REALLY_USED_REGISTERS</code>.  However, some ABIs specify that calls
must preserve certain bits of a particular register but can alter others.
This hook should return true if this applies to at least one of the
registers in &lsquo;<samp>(reg:<var>mode</var> <var>regno</var>)</samp>&rsquo;, and if as a result the
call would alter part of the <var>mode</var> value.  For example, if a call
preserves the low 32 bits of a 64-bit hard register <var>regno</var> but can
clobber the upper 32 bits, this hook should return true for a 64-bit mode
but false for a 32-bit mode.
</p>
<p>The value of <var>abi_id</var> comes from the <code>predefined_function_abi</code>
structure that describes the ABI of the call; see the definition of the
structure for more details.  If (as is usual) the target uses the same ABI
for all functions in a translation unit, <var>abi_id</var> is always 0.
</p>
<p>The default implementation returns false, which is correct
for targets that don&rsquo;t have partly call-clobbered registers.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fGET_005fMULTILIB_005fABI_005fNAME"></a>Target Hook: <em>const char *</em> <strong>TARGET_GET_MULTILIB_ABI_NAME</strong> <em>(void)</em></dt>
<dd><p>This hook returns name of multilib ABI name.
</p></dd></dl>

<a name="index-fixed_005fregs"></a>
<a name="index-call_005fused_005fregs"></a>
<a name="index-global_005fregs"></a>
<a name="index-reg_005fnames"></a>
<a name="index-reg_005fclass_005fcontents"></a>
<dl>
<dt><a name="index-TARGET_005fCONDITIONAL_005fREGISTER_005fUSAGE"></a>Target Hook: <em>void</em> <strong>TARGET_CONDITIONAL_REGISTER_USAGE</strong> <em>(void)</em></dt>
<dd><p>This hook may conditionally modify five variables
<code>fixed_regs</code>, <code>call_used_regs</code>, <code>global_regs</code>,
<code>reg_names</code>, and <code>reg_class_contents</code>, to take into account
any dependence of these register sets on target flags.  The first three
of these are of type <code>char []</code> (interpreted as boolean vectors).
<code>global_regs</code> is a <code>const char *[]</code>, and
<code>reg_class_contents</code> is a <code>HARD_REG_SET</code>.  Before the macro is
called, <code>fixed_regs</code>, <code>call_used_regs</code>,
<code>reg_class_contents</code>, and <code>reg_names</code> have been initialized
from <code>FIXED_REGISTERS</code>, <code>CALL_USED_REGISTERS</code>,
<code>REG_CLASS_CONTENTS</code>, and <code>REGISTER_NAMES</code>, respectively.
<code>global_regs</code> has been cleared, and any <samp>-ffixed-<var>reg</var></samp>,
<samp>-fcall-used-<var>reg</var></samp> and <samp>-fcall-saved-<var>reg</var></samp>
command options have been applied.
</p>
<a name="index-disabling-certain-registers"></a>
<a name="index-controlling-register-usage"></a>
<p>If the usage of an entire class of registers depends on the target
flags, you may indicate this to GCC by using this macro to modify
<code>fixed_regs</code> and <code>call_used_regs</code> to 1 for each of the
registers in the classes which should not be used by GCC.  Also make
<code>define_register_constraint</code>s return <code>NO_REGS</code> for constraints
that shouldn&rsquo;t be used.
</p>
<p>(However, if this class is not included in <code>GENERAL_REGS</code> and all
of the insn patterns whose constraints permit this class are
controlled by target switches, then GCC will automatically avoid using
these registers when the target switches are opposed to them.)
</p></dd></dl>

<dl>
<dt><a name="index-INCOMING_005fREGNO"></a>Macro: <strong>INCOMING_REGNO</strong> <em>(<var>out</var>)</em></dt>
<dd><p>Define this macro if the target machine has register windows.  This C
expression returns the register number as seen by the called function
corresponding to the register number <var>out</var> as seen by the calling
function.  Return <var>out</var> if register number <var>out</var> is not an
outbound register.
</p></dd></dl>

<dl>
<dt><a name="index-OUTGOING_005fREGNO"></a>Macro: <strong>OUTGOING_REGNO</strong> <em>(<var>in</var>)</em></dt>
<dd><p>Define this macro if the target machine has register windows.  This C
expression returns the register number as seen by the calling function
corresponding to the register number <var>in</var> as seen by the called
function.  Return <var>in</var> if register number <var>in</var> is not an inbound
register.
</p></dd></dl>

<dl>
<dt><a name="index-LOCAL_005fREGNO"></a>Macro: <strong>LOCAL_REGNO</strong> <em>(<var>regno</var>)</em></dt>
<dd><p>Define this macro if the target machine has register windows.  This C
expression returns true if the register is call-saved but is in the
register window.  Unlike most call-saved registers, such registers
need not be explicitly restored on function exit or during non-local
gotos.
</p></dd></dl>

<dl>
<dt><a name="index-PC_005fREGNUM"></a>Macro: <strong>PC_REGNUM</strong></dt>
<dd><p>If the program counter has a register number, define this as that
register number.  Otherwise, do not define it.
</p></dd></dl>

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